Pixel bistability is a desirable attribute for a liquid crystal display (‘LCD’) because this eliminates the need constantly to refresh the display or to employ a silicon memory device behind each pixel, which becomes prohibitively expensive as the number of pixels increases. With bistability, only pixels that need to be changed need addressing, and simple matrix addressing may be employed. If the liquid crystal (‘LC’) can switch at video frame rates, the device may be used as an LCD TV or monitor. For other applications such as ‘digital paper’, where the device is used as a permanent or semi-permanent display for information which can be changed or updated from time to time, slower switching times are acceptable.
In co-pending U.S. patent application Ser. No. 10/698,028, electrophoretically-controlled nematic (EPCN) bistable devices are presented in which a ‘hidden’ electrophoretic effect is used to stabilise the LC in one of two stable states. The nematic LC is doped with nanoparticles carrying either a positive or a negative charge. An optical effect is observed by using a homeoplanar or 90° twist cell. The homeoplanar cell has a homeotropic alignment on one substrate and a planar alignment on an opposed substrate. The alignments are provided using different materials and different fabrication techniques, requiring some complexity in the process for fabricating the device. The 90° twisted cell switches between twist and homeoplanar states, and requires electrical pulses that control switching close to one surface. Such switching may, however, require very precise control of the electrical pulses.